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How to Think Like a Computer Scientist

Chapter 7

Strings

7.1 A compound data type

So far we have seen three types: int, float, and string. Strings are qualitatively different from the
other two because they are made up of smaller pieces characters.

Types that comprise smaller pieces are called compound data
types. Depending on what we are doing, we may want to treat a
compound data type as a single thing, or we may want to access its
parts. This ambiguity is useful.

The bracket operator selects a single character from a string.

>>> fruit = "banana">>> letter = fruit[1]
>>> print letter

The expression fruit[1] selects character number 1 from fruit. The variable letter refers to the result. When we
display letter, we get a surprise:

a

The first letter of "banana" is not a. Unless you are a
computer scientist. In that case you should think of the expression in
brackets as an offset from the beginning of the string, and the offset
of the first letter is zero. So b is the 0th letter
("zero-eth") of "banana", a is the 1th letter
("one-eth"), and n is the 2th ("two-eth") letter.

To get the first letter of a string, you just put 0, or
any expression with the value 0, in the brackets:

>>> letter = fruit[0]
>>> print letter
b

The expression in brackets is called an index. An index
specifies a member of an ordered set, in this case the set of
characters in the string. The index indicates which one you
want, hence the name. It can be any integer expression.

7.2 Length

The len function returns the number of characters in a string:

>>> fruit = "banana">>> len(fruit)
6

To get the last letter of a string, you might be tempted to try something
like this:

length = len(fruit)
last = fruit[length] # ERROR!

That won't work. It causes the runtime error IndexError: string
index out of range. The reason is that there is no 6th letter in
"banana". Since we started counting at zero, the six letters
are numbered 0 to 5. To get the last character, we have to subtract
1 from length:

length = len(fruit)
last = fruit[length-1]

Alternatively, we can use negative indices, which count backward from the end
of the string. The expression fruit[-1] yields the last letter,
fruit[-2] yields the second to last, and so on.

7.3 Traversal and the for loop

A lot of computations involve processing a string one character at a
time. Often they start at the beginning, select each character in
turn, do something to it, and continue until the end. This pattern of
processing is called a traversal. One way to encode a traversal
is with a while statement:

This loop traverses the string and displays each letter on a line by
itself. The loop condition is index < len(fruit), so
when index is equal to the length of the string, the
condition is false, and the body of the loop is not executed. The
last character accessed is the one with the index len(fruit)-1,
which is the last character in the string.

As an exercise, write a function that takes a string as an argument
and outputs the letters backward, one per line.

Using an index to
traverse a set of values is so common that
Python provides an alternative, simpler syntax the for loop:

for char in fruit:
print char

Each time through the loop, the next character in the string is assigned
to the variable char. The loop continues until no characters are
left.

The following example shows how to use concatenation and a for loop to generate an abecedarian series. "Abecedarian" refers
to a series or list in which the elements appear in alphabetical
order. For example, in Robert McCloskey's book Make Way for
Ducklings, the names of the ducklings are Jack, Kack, Lack, Mack,
Nack, Ouack, Pack, and Quack. This loop outputs these names in order:

prefixes = "JKLMNOPQ"suffix = "ack"

for letter in prefixes:
print letter + suffix

The output of this program is:

Jack
Kack
Lack
Mack
Nack
Oack
Pack
Qack

Of course, that's not quite right because "Ouack" and
"Quack" are misspelled.

As an exercise, modify the program to fix this error.

7.4 String slices

A segment of a string is called a
slice. Selecting a slice is similar to
selecting a character:

The operator [n:m] returns the part of the string from the
"n-eth" character to the "m-eth" character, including the first but
excluding the last. This behavior is counterintuitive; it makes
more sense if you imagine the indices pointing between the
characters, as in the following diagram:

If you omit the first index (before the colon), the slice starts at the
beginning of the string. If you omit the second index, the slice goes to the
end of the string. Thus:

>>> fruit = "banana">>> fruit[:3]
'ban'>>> fruit[3:]
'ana'

What do you think s[:] means?

7.5 String comparison

The comparison operators work on
strings. To see if two strings are equal:

if word == "banana":
print"Yes, we have no bananas!"

Other comparison operations are useful for putting words in alphabetical
order:

You should be aware, though, that Python does not handle upper-
and lowercase letters the same way that people do. All the uppercase
letters come before all the lowercase letters. As a result:

Your word, Zebra, comes before banana.

A common way to address this problem is to convert strings to a standard
format, such as all lowercase, before performing the comparison. A more
difficult problem is making the program realize that zebras are not fruit.

7.6 Strings are immutable

It is tempting to use the [] operator on the left side of an
assignment, with the intention of changing a character in a string.
For example:

7.7 A find function

In a sense, find is the opposite of the [] operator.
Instead of taking an index and extracting the corresponding character,
it takes a character and finds the index where that character
appears. If the character is not found, the function returns -1.

This is the first example we have seen of a return statement
inside a loop.
If str[index] == ch, the function returns
immediately, breaking out of the loop prematurely.

If the character doesn't appear in the string, then the program
exits the loop normally and
returns -1.

This pattern of computation is sometimes called a "eureka" traversal
because as soon as we find what we are looking for, we can cry
"Eureka!" and stop looking.

As an exercise, modify the find function so that it has a
third parameter, the index in the string where it should start
looking.

7.8 Looping and counting

The following program counts the number of times the letter a
appears in a string:

This program demonstrates another pattern of computation called a counter. The variable count is initialized to 0 and then
incremented each time an a is found. (To increment is to
increase by one; it is the opposite of decrement, and unrelated
to "excrement," which is a noun.) When the loop exits, count
contains the result the total number of a's.

As an exercise, encapsulate this code in a function named countLetters, and generalize it so that it accepts the string and the
letter as arguments.

As a second exercise, rewrite this function so that instead of
traversing the string, it uses the three-parameter version of find from the previous.

7.9 The string module

The string module contains useful functions that
manipulate strings. As usual, we have to import the module before
we can use it:

>>> import string

The string module includes a function named find that does
the same thing as the function we wrote. To call it we have to
specify the name of the module and the name of the function using dot
notation.

This example demonstrates one of the benefits of modules they help
avoid collisions between the names of built-in functions and
user-defined functions. By using dot notation we can specify which
version of find we want.

Actually, string.find is more general than our version. First,
it can find substrings, not just characters:

>>> string.find("banana", "na")
2

Also, it takes an additional argument that specifies the index it
should start at:

>>> string.find("banana", "na", 3)
4

Or it can take two additional arguments that specify a range
of indices:

>>> string.find("bob", "b", 1, 2)
-1

In this example, the search fails because the letter b does not
appear in the index range from 1 to 2 (not including 2).

7.10 Character classification

It is often helpful to examine a character and test whether it is
upper- or lowercase, or whether it is a character or a digit. The
string module provides several constants that are
useful for these purposes.

The string string.lowercase contains all of the letters that the
system considers to be lowercase. Similarly, string.uppercase
contains all of the uppercase letters. Try the following and see what
you get:

We can use these constants and find to classify characters. For
example, if find(lowercase, ch) returns a value other than -1, then ch must be lowercase:

defisLower(ch):
return string.find(string.lowercase, ch) != -1

Alternatively, we can take advantage of the in operator, which
determines whether a character appears in a string:

defisLower(ch):
return ch in string.lowercase

As yet another alternative, we can use the comparison operator:

defisLower(ch):
return'a' <= ch <= 'z'

If ch is between a and z, it must be a lowercase
letter.

As an exercise, discuss which version of isLower you think
will be fastest. Can you think of other reasons besides speed to
prefer one or the other?

Another constant defined in the string module may
surprise you when you print it:

>>> print string.whitespace

Whitespace characters move the cursor without printing
anything. They create the white space between visible
characters (at least on white paper). The constant
string.whitespace contains all the
whitespace characters, including
space, tab (\t), and newline
(\n).

There are other useful functions in the string module, but this
book isn't intended to be a reference manual. On the other hand, the
Python Library Reference is. Along with a wealth of other
documentation, it's available from the Python website, www.python.org.

7.11 Glossary

compound data type

A data type in which the values are made
up of components, or elements, that are themselves values.

traverse

To iterate through the elements of a set,
performing a similar operation on each.

index

A variable or value used to select a member of an
ordered set, such as a character from a string.

slice

A part of a string specified by a range of indices.

mutable

A compound data types whose elements can be assigned
new values.

counter

A variable used to count something, usually initialized
to zero and then incremented.

increment

To increase the value of a variable by one.

decrement

To decrease the value of a variable by one.

whitespace

Any of the characters that move the cursor without
printing visible characters. The constant
string.whitespace
contains all the whitespace characters.

Warning: the HTML version of this document is generated from
Latex and may contain translation errors. In
particular, some mathematical expressions are not translated correctly.